The present invention relates to a plasma source apparatus utilizing microwave power and, more particularly, to a microwave plasma source apparatus capable of producing a high-temperature, high-density plasma with high efficiency and stability for use in trace element spectrometry with plasma source, plasma jet and plasma processing.
Prior art plasma source apparatus that utilize microwave power are illustratively discussed in such publications as "Spectrochimica Acta (Vol. 37B, No. 7, pp. 583-592, 1982)."
FIGS. 6A and 6B outline the microwave plasma source apparatus called SURFATRON which is described in the above-mentioned publication. This apparatus has its microwave power supplied through a coaxial connector 1 which is connected to a coaxial cable. The supplied microwave power passes through a coaxial line before being introduced into a cavity 6 via a microwave coupler 2. The cavity 6 is penetrated by a quartz discharge tube 7. At one axial end of the cavity 6 is a metal plate 4. The axial length of the cavity 6 is adjusted by a cavity length adjusting system 5. A gap length "g" is adjusted by a gap adjusting system 3. Cooling air is introduced into the cavity 6 through an air introduction port 9.
In this apparatus, a sample is mixed with a carrier gas for use as a gas sample. The gas sample is mixed with a plasma gas for introduction into the discharge tube 7 through a gas introduction port 8. The plasma gas thus introduced is excited by the microwave power led into the cavity 6, thereby producing a plasma inside the discharge tube 7. In this plasma, the sample is excited and then ionized.
One disadvantage of this prior art apparatus is that it has no consideration for analysis of aqueous samples. The fact that the apparatus accepts only gas samples for analysis limits the types of samples to be analyzed. Another disadvantage of the prior art apparatus is its low levels of sample introduction efficiency and ionization efficiency. A further disadvantage is that the apparatus does not provide an adequate safeguard against leaks of microwave power. Another disadvantage is that the stability of plasma production with this apparatus is not sufficiently high.
As shown in FIG. 6A, the prior art apparatus supplies the cavity 6 with microwave power for plasma production through the coaxial cable and coupler 2. This arrangement limits the level of the plasma power that may be supplied to a maximum of about 0.5 kW. Thus the apparatus is incapable of producing a hot and high density plasma directly excite and ionize aqueous samples. This means that direct analysis of aqueous samples is impossible with the prior art apparatus. Other impediments include an appreciable loss of power over the coaxial cable, complicated structures of such components as coupler 2, and cumbersome adjustments required of these parts. Because the plasma source apparatus of the prior art construction has low levels of efficiency in utilizing microwave power, the apparatus when used as an ionizer for analytical samples fails to provide sufficiently high levels of sensitivity due to its low levels of ionization efficiency.
Furthermore, the leaking microwave power from openings such as a window of the metal plate 4 at one end of the cavity 6 tends to worsen the S/N ratio of signals detected for sample analysis. The leaks can also cause jamming of radio waves. The air introduced to cool the discharge tube 7 and other related parts flows from the window of the metal plate 4 toward the tip of the discharge tube 7. The air flow destabilizes the state of the plasma released from the tip of the discharge tube 7. This makes it impossible to ensure high sensitivity and stability in carrying out emission analyses using the emission of samples or in conducting mass analyses using sample ionization.